The present invention relates to a crystal for a telephone watch including a keyboard formed in particular of a plurality of capacitive sensors.
A watch crystal under which there is arranged a keyboard formed of a plurality of capacitive sensors has already been proposed on several occasions. European Patent No. EP 0 674 247 will be taken here by way of example. The watch disclosed includes a case, a crystal and at least one manual control device including a capacitive sensor provided with an electrode disposed on the inner face of the crystal. Selective positioning of a finger of the wearer of the watch on the outer face of the crystal allows a capacitance to be formed between the electrode and earth formed by the watch case. This manual control device also includes a voltage-frequency converter whose oscillation frequency is determined by the aforementioned capacitance. The electrode is connected by a conductor to the converter which is housed in the case.
The keyboard in question may be intended to replace the usual external control means such as push-buttons used to control the various functions of a watch, such as time-setting or starting and stopping a chronograph. It will be understood however that this keyboard may also be used as a selector for dialling a telephone number if the watch is provided with a radio telephone.
The telephone watch which will be discussed hereinafter preferably has a case made of plastic material on which a crystal, also made of plastic material, is mounted. This crystal, which is transparent, allows the hour and minute hands to be seen at its centre and the numbers and signs at its periphery, these numbers and signs serving both as time indices and telephone dial numbers. The plastic crystals of currently known watches generally have a sufficient thickness, on the one hand, to resist shocks from external agents and on the other to bear a certain hydrostatic pressure when the watch is immersed in water. The designer has set himself a limit of 3 bars for such pressure here. It will therefore be understood that these constraints mean that a crystal of sufficiently large thickness has to be used which prevents the proper operation of a keyboard placed under the crystal. Indeed, a thick crystal leads to a significant dielectric interposed between the electrodes of the capacitor formed by the sensor and the user's finger. Thus, the capacitance variation may be low, or even insignificant if the dielectric is large.